Influence of temperature during pyrolysis of Fe-alginate: Unraveling the pathway towards highly active Fe/C catalysts

Transition metals supported on carbons play an important role in catalysis and energy storage. By pyrolysis of metal alginate, highly active catalysts for the Fischer-Tropsch synthesis (FTS) can be produced. However, the evolution of the carbon (alginate) and transition metal (Fe3+) during pyrolysis remains largely unknown and was herein corroborated with several advanced in situ techniques. Initially, Fe3+ was reduced to Fe2+, while bound to alginate. FeO nucleated above 300 °C, destabilizing the alginate functional groups. Increasing temperatures improved carbonization of the carbon support, which facilitated reduction of FeO to α-Fe at 630 °C. Catalysts were produced by pyrolysis between 400 and 700 °C, where the highest FTS activity (612 µmolCO gFe−1 s−1) was achieved for the sample pyrolyzed at low temperature. Lower metal loading, due to less decomposition of alginate, moderated sintering and yielded larger catalytic surface areas. The results provide valuable knowledge for rational design of metal-alginate-based materials.publishedVersio

The impact of sequential H2-CO-H2 activation treatment on the structure and performance of cobalt based catalysts for the Fischer-Tropsch synthesis

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Catalyst Deactivation During One-Step Dimethyl Ether Synthesis from Synthesis Gas

Catalysts for direct synthesis of dimethyl ether (DME) from synthesis gas should essentially contain two functions, i.e., methanol synthesis and methanol dehydration. In the present work, the deactivation of both functions of hybrid catalysts during direct DME synthesis under industrially relevant conditions has been investigated with special focus on the influence of each reaction step on the deactivation of the catalyst function corresponding to the other step. A physical mixture of a Cu–Zn-based methanol synthesis catalyst and a ZSM-5 methanol dehydration catalyst was used. The metallic catalyst appears to deactivate due to Cu sintering, with no apparent effect from the methanol dehydration step under the conditions applied. The acid catalyst deactivates due to accumulation of hydrocarbon species formed in its pores. Synthesis gas composition, i.e., {H}2/CO ratio and {CO}2-content (which directly affects partial pressure of water), seems to influence the zeolite deactivation.acceptedVersio

Direct dimethyl ether synthesis from synthesis gas: The influence of methanol dehydration on methanol synthesis reaction

Direct dimethyl ether (DME) synthesis from synthesis gas is studied with regard to potential effects of methanol dehydration on methanol formation and copper-based catalyst performance. For this, the influence of the operating conditions (space velocity, temperature, pressure, time-on-stream and syngas composition) on activity, selectivity and stability of the catalyst was studied and compared for methanol synthesis and direct DME synthesis. The advantage of the direct over the two-step DME synthesis is apparent at conditions where syngas conversion to methanol is thermodynamically limited. However, under the applied operating conditions, results suggest that combining methanol synthesis and dehydration has a negative effect on the methanol formation kinetics. The origin of the observed phenomena is investigated by varying dehydration catalyst and by introducing dehydration products (DME and water) into the methanol synthesis feed. Choice of the solid acid catalyst does not seem to affect methanol formation, and DME is also found to be practically inert over the methanol synthesis catalysts. Water injection, on the other hand, led to a significant decrease in the methanol synthesis rate. Thus, formation of an additional amount of water through methanol dehydration might be an explanation for the lower methanol formation rate in the direct DME synthesis.acceptedVersio

Fischer-Tropsch synthesis—Investigation of the deactivation of a Co catalyst by exposure to aerosol particles of potassium salt

The influence of potassium species on a Co based Fischer-Tropsch catalyst was investigated using an aerosol deposition technique. This way of poisoning the catalyst was chosen to simulate the actual potassium behaviour during the biomass to liquid (BTL) process utilizing gasification followed by fuel synthesis. A reference catalyst was poisoned with three levels of potassium and the samples were characterized and tested for the Fischer- Tropsch reaction under industrially relevant conditions. None of the conventional characterization techniques applied (H2 Chemisorption, BET, TPR) divulged any difference between poisoned and unpoisoned samples, whereas the activity measurements showed a dramatic drop in activity following potassium deposition. The results are compared to previous results where incipient wetness impregnation was used as the method of potassium deposition. The effect of potassium is quite similar in the two cases, indicating that irrespective of how potassium is introduced it will end up in the same form and on the same location on the active surface. This indicates that potassium is mobile under FTS conditions, and that potassium species are able to migrate to sites of particular relevance for the FT reaction.acceptedVersio

The effect of aerosol-deposited ash components on a cobalt-based Fischer–Tropsch catalyst

Postprint version of published articleThe effect of ash salts on Co-based Fisher–Tropsch catalysts was studied using an aerosol deposition technique. The major elements in the ash were found to be K, S and Cl. The ash was deposited on a calcined catalyst as dry particles with an average diameter of approx. 350 nm. The loading of ash particles was varied by varying the time of exposure to the particles in a gas stream. Catalyst characterization did not reveal significant differences in cobalt dispersion, reducibility, surface area, pore size, or pore volume between the reference and the catalysts with ash particles deposited. Activity measurements showed that following a short exposure to the mixed ash salts (30 min), there were no significant loss of activity, but a minor change in selectivity of the catalyst . Extended exposure (60 min) led to some activity loss and changes in selectivity. However, extending the exposure time and thus the amount deposited as evidenced by elemental analysis did not lead to a further drop in activity. This behavior is different from that observed with pure potassium salts, and is suggested to be related to the larger size of the aerosol particles deposited. The large aerosol particles used here were probably not penetrating the catalyst bed, and to some extent formed an external layer on the catalyst bed. The ash salts are therefore not able to penetrate to the pore structure and reach the Co active centers, but are mixed with the catalyst and detected in the elemental analysis.acceptedVersio

The effect of aerosol-deposited ash components on a cobalt-based Fischer–Tropsch catalyst

Postprint version of published articleThe effect of ash salts on Co-based Fisher–Tropsch catalysts was studied using an aerosol deposition technique. The major elements in the ash were found to be K, S and Cl. The ash was deposited on a calcined catalyst as dry particles with an average diameter of approx. 350 nm. The loading of ash particles was varied by varying the time of exposure to the particles in a gas stream. Catalyst characterization did not reveal significant differences in cobalt dispersion, reducibility, surface area, pore size, or pore volume between the reference and the catalysts with ash particles deposited. Activity measurements showed that following a short exposure to the mixed ash salts (30 min), there were no significant loss of activity, but a minor change in selectivity of the catalyst . Extended exposure (60 min) led to some activity loss and changes in selectivity. However, extending the exposure time and thus the amount deposited as evidenced by elemental analysis did not lead to a further drop in activity. This behavior is different from that observed with pure potassium salts, and is suggested to be related to the larger size of the aerosol particles deposited. The large aerosol particles used here were probably not penetrating the catalyst bed, and to some extent formed an external layer on the catalyst bed. The ash salts are therefore not able to penetrate to the pore structure and reach the Co active centers, but are mixed with the catalyst and detected in the elemental analysis.acceptedVersio

The impact of sequential H2-CO-H2 activation treatment on the structure and performance of cobalt based catalysts for the Fischer-Tropsch synthesis

Different activation protocols were applied to a promoted Re/Co/γ-Al2O3 catalyst for Fischer-Tropsch synthesis. The activation included treatment in either pure H2 or a sequence of Reduction (H2) – Carburization (CO) – Reduction (H2) in order to vary the microstructure of Co nanoparticles and investigate their performance. The alternative activation protocol applied under specific conditions (carburization at 230 °C and final reduction at 350 °C) outperform the conventional activation both in terms of catalyst activity and C5+ selectivity. The catalyst was characterized in all the stages of the activation process by synchrotron based X-ray diffraction (XRD) and X-ray absorption near edge structure spectroscopy (XANES). The effect of air exposure at room temperature of the carburized sample was investigated and it was found that air exposure leads to minor phase changes that significantly affect catalyst performance. Carburization temperature and temperature used for the decomposition of the carbide on the final reduction step both had a severe effect on catalyst performance, but neither of them influenced considerably the microstructure of Co nanoparticles that was primarily hexagonal (hcp). All carburized samples contained a significant amount of carbon as observed by Thermogravimetric analysis (TGA) and Raman spectroscopy. The final reduction step could remove part of the excess carbon produced during CO disproportionation. It appears that the carbon removal\minimization is a key factor for the exploitation of the benefits of hcp configuration of Co nanoparticles catalysing Fischer-Tropsch synthesis

The Effect of Copper Loading on Iron Carbide Formation and Surface Species in Iron‐Based Fischer–Tropsch Synthesis Catalysts

This is the accepted version of the following article: Peña, D., Jensen, L., Cognigni, A., Myrstad, R., Neumayer, T., Van Beek, W., & Rønning, M. (2018). The Effect of Copper Loading on Iron Carbide Formation and Surface Species in Iron‐Based Fischer–Tropsch Synthesis Catalysts. ChemCatChem, 10(6), 1300-1312., which has been published in final form at http://dx.doi.org/10.1002/cctc.201701673.The effect of copper as promoter on iron carbide formation and the nature of surface species on iron‐based catalyst during Fischer–Tropsch synthesis (FTS) was investigated. Iron‐based catalysts (15 wt % of Fe) supported on alumina promoted with copper (0, 0.6, 2, and 5 wt %) were characterised in situ at relevant FTS conditions. The catalysts promoted with 2 and 5 wt % of Cu showed higher catalytic activity due to the formation of Hägg carbide (Fe5C2) detected by in situ XANES and XRD. The carbide formation is attributed to a weakening of the iron–alumina mixed‐compound interactions, and hence increasing iron reducibility and dispersion. The in situ XANES measurements indicated a maximum carburization degree (ca. 20–22 %) even at high Cu loading. The catalyst promoted with 5 wt % of Cu exhibited higher water‐gas shift activity. Aliphatic hydrocarbons, formate, and carboxylate species were detected on the catalyst surface during FTS. After exposing the spent catalysts to hydrogenation conditions, the carboxylate species remained strongly adsorbed while aliphatic hydrocarbons and formate species disappeared. The accumulation of oxygenates species on the catalyst surface increased with Cu loading. The interaction of oxygenates with alumina and iron oxide particles (FexOy) were revealed, with the latter being a possible reason for inhibition of further iron carburization.acceptedVersio

Transition-Metal Nanoparticle Catalysts Anchored on Carbon Supports via Short-Chain Alginate Linkers

This study reports a green, inexpensive, and highly versatile procedure to synthesize well-dispersed transition-metal nanoparticles anchored on carbon supports. The resulting metal loadings are 26 wt % or above. Achieving both these properties simultaneously has been difficult with established synthesis methods of carbon-supported metal catalysts, such as impregnation and deposition-precipitation. Herein, low-molar-mass sodium alginate with high guluronate content was ion-exchanged with transition-metal ions, followed by a pyrolysis step at 500 °C. The investigated transition-metal ions were Fe3+, Co2+, Ni2+, and Cu2+. The alginate’s properties and interaction with the transition-metal ions greatly influenced the pyrolyzed material’s characteristics, whereas the observed metal particle size was found to negatively correlate with the metal’s melting point. The pyrolyzed Fe-alginate was tested as a catalyst for the Fischer−Tropsch synthesis and exhibited an iron time yield of 885 μmolCO h−1 g−1 , which is among the highest activities reported in the literature. The activity is mainly attributed to the iron nanoparticle size achieved by the reported synthesis procedure, and the improved olefin selectivity is ascribed to the sodium and sulfur that originates from the alginate and iron precursor, respectively.publishedVersio